1. Field of the Invention
The present invention relates to inductors and, more particularly, to a semiconductor die with an integrated inductor.
2. Description of the Related Art
Inductors are common circuit elements that are used in numerous devices. In many applications, inductors are implemented as stand-alone devices which, in turn, consume a significant amount of circuit board space. In order to minimize the required circuit board space for hand-held devices, it is desirable to integrate an inductor into a chip.
FIGS. 1A-1D shows views that illustrate a prior art integrated circuit inductor 100. FIG. 1A shows a plan view, while FIG. 1B shows a cross-sectional view taken along lines 1B-1B of FIG. 1A. In addition, FIG. 1C shows a cross-sectional view taken along lines 1C-1C of FIG. 1A, while FIG. 1D shows a cross-sectional view taken along lines 1D-1D of FIG. 1A.
As shown in FIGS. 1A-1D, inductor 100 is formed on top of a four metal layer interconnect structure that includes a fourth layer of insulation material I4, and a metal trace 110 that is formed on insulation layer I4 from a fourth metal layer M4. In addition, the metal interconnect structure includes a fifth layer of insulation material I5 that is formed on metal trace 110, and a via 112 that is formed through insulation layer I5 to make an electrical connection with metal trace 110.
As further shown in FIGS. 1A-1D, inductor 100 includes a metal trace 114 that is formed on top of the fifth layer of insulation material I5 from a fifth metal layer M5. Metal trace 114, which has a width W and a depth D, has a first end 120 that is formed over via 112 to make an electrical connection with via 112, and a second end 122. Metal trace 114, which makes one and a half loops in the same plane, is typically formed on top of the metal interconnect structure to avoid inducing currents in the substrate.
One important measure of an inductor is the quality factor or Q of an inductor. High Q inductors are desirable in a number of RF circuits, such as resonant circuits. The Q of an inductor is a measure of the ratio of magnetic energy stored in the inductor versus the total energy fed into the inductor, and is given by equation (EQ.) 1 as:Q=ωL/Z,  EQ. 1where ω is related to the frequency f of the signal applied to the inductor (ω=2(pi)(f)), L represents the inductance of the inductor, and Z represents the RF impedance of the inductor. Thus, as indicated by EQ. 1, the higher the inductance, the higher the Q of the inductor.
One approach to increasing the inductance of an inductor is to form the coil structure around a ferromagnetic core structure. Due to the difficulty in forming planar inductors, such as inductor 100, around a ferromagnetic core structure, many current-generation integrated circuit inductors are formed as micro-electromechanical system (MEMS) devices.
FIG. 2 shows a perspective view that illustrates an example of a prior-art MEMS inductor 200. As shown in FIG. 2, MEMS inductor 200 includes a base conductive plate 210, and a top conductive plate 212 that lies over base conductive plate 210. MEMS inductor 200 also includes a conductive sidewall 214 that has a bottom surface that contacts base conductive plate 210, and a top surface that contacts top conductive plate 212. In addition, MEMS inductor 200 further includes a conductive sidewall 216 that has a top surface that contacts top conductive plate 212.
As shown, base conductive plate 210, top conductive plate 212, and conductive sidewalls 214 and 216, which can be formed from materials including copper, define an enclosed region 220 that lies only between the base and top conductive plates 210 and 212, and sidewalls 214 and 216.
As further shown, MEMS inductor 200 includes a magnetic core structure 222 that is located within enclosed region 220, and within no other enclosed regions. Magnetic core structure 222, which is electrically isolated from all other conductive regions, can be implemented in a number of prior-art fashions. For example, magnetic core structure 222 can be implemented with a number of laminated Ni—Fe cores 224 which are thin enough to minimize eddy currents.
In operation, a current I1 can flow into MEMS inductor 200 along the bottom side of sidewall 216, and out along the near end of bottom conductive plate 210 that lies away from sidewall 214. A current I2 can also flow in the opposite direction, flowing into MEMS inductor 200 along the end of bottom conductive plate 210 that lies away from sidewall 214, and flowing out along the bottom side of sidewall 216.
Although there are various solutions for forming integrated circuit inductors around ferromagnetic core structures, there is a continuing need for integrated circuit inductors which are simple to form and which provide a larger inductance that can be obtained with conventional planar inductors.